Fix file encoding migration

Summary:
See Discourse

When migrating to aes-256-cbc, integrity hashes were not updated, so data was not properly

Test Plan:
I ran this test script to ensure it worked.
I created some files with lipsum, ensured that after encoding them with aes-256-cbc, they were not able to be cat'd.
After applying this patch and rerunning the script, it worked successfully.

Reviewers: epriestley, amckinley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin

Tags: #files, #storage

Differential Revision: https://secure.phabricator.com/D19533